In dynamic type memory devices (hereafter, referred to as DRAM), a refreshing operation is carried out such that stored information is read out in a predetermined time period before the stored information disappears, and the information is amplified and thereafter rewritten into memory cells. As the refreshing operation, there are the RAS only refreshing mode with employment of the row-address strobe signal "RAS" and the CBR refreshing mode in which the column-address strobe signal "CAS" is set to a low level in advance before the row-address strobe signal "RAS" is set to a low level. The dynamic type RAMs capable of such a refreshing mode are known from, for instance, JP-A-3-14669 laid-open on Sep. 19, 1991.